1. Field of the Invention
The present invention relates to a method for forming bumps and a semiconductor device obtained therefrom. Specifically, the present invention relates to a method, for forming bumps, using a transfer technology and capable of improving a yield of semiconductor devices, and a semiconductor device obtained therefrom.
2. Description of the Related Art
Recent semiconductor devices are adapted so that a surface of a semiconductor substrate, for example, of silicon on which circuit patterns are provided, is directly mounted to a corresponding surface of a printed circuit board in order to prevent signal transmission from being delayed and to ensure that the integration density is enhanced. For this purpose, projections called bumps are formed on electrodes of the semiconductor substrate, and the mounting of the semiconductor substrate onto the printed circuit board is carried out via the bumps, whereby the difference in height between the respective electrodes on the semiconductor substrate and between those on the printed circuit board is absorbed by the bumps.
Such bumps have been formed by a method called a wire-bonding. This, however, has a drawback in accordance with the increase in the number of terminals per chip in that the time and/or the cost for the production of bumps significantly increases because the bumps must be formed one by one in the wire-bonding method. Generally speaking, the cost for forming the bumps must be restricted in a range from 1 to 0.01% of that of a chip, and if the cost exceeds this allowable value, it is practically impossible to produce the semiconductor devices. To solve such a problem, in a case of a relatively large size chip, electrodes which have been formed solely in the peripheral area of the chip in the prior art are provided all over the chip to lower the installation density of the electrodes and increase the distance between adjacent electrodes. This allows forming bumps by vapor deposition, plating or printing which provide the bumps at once on all of the electrodes before a wafer is diced into individual semiconductor substrates carrying circuit patterns thereon, to result in a reduction in the production cost.
However, there has been a demand for forming bumps even on a chip having a smaller electrode pitch (i.e., a higher electrode density) due to the recent tendency to high performance and high density in semiconductor devices. In response thereto, a method for forming bumps using a transfer technology (hereinafter referred to a transfer method) has been proposed, for example, in Japanese Unexamined Patent Publication Nos. 1-189942 and 3-190138. When a vapor deposition, plating or printing method is used, it is necessary to select solder materials and production processes so that no adverse effect occurs on the semiconductor substrate, which means that the use of novel solder materials or novel production processes may be difficult.
For example, in a case of vapor deposition, materials are limited to those capable of being vapor-deposited, and a fluorine type gas, which damages a semiconductor substrate surface, is not usable. Also, in a case of plating, materials are limited to those capable of being plated, and a plating solution which damages a semiconductor substrate is not usable. In a case of printing, materials are limited to those capable of being printed, and a large pressure or a flux which damages a semiconductor substrate is not usable. Contrarily, according to the transfer method, such restrictions are eliminated, and it is possible to freely employ low cost materials or production processes and/or novel materials or production processes from which a high yield is achievable. As a result, bumps can be formed at high density by a transfer method while maintaining a favorable yield substantially equal to that of a semiconductor substrate having a low electrode density obtained by a conventional method such as a vapor deposition or others.
The production yield of the bumps is at most 99.999% even if the above-mentioned transfer method is used, which means that this method is usable for the production of chips of a highest specification now marketed, each having 1000 bumps, in view of the production cost, but is not applicable to the production of chips, each having 2000 to 8000 bumps. Further, if the transfer method is used for the production of chips having 10,000 bumps or more, such as those used for the research and development, the production yield per chip is at most 90%. Particularly, since the mass-production technique is hardly applicable to the production of the chips used for the research and development, the production cost becomes higher as well as the production yield becomes lower, and there is a case, for example, at an initial stage of the research, wherein only 1% of the products are non-defective. Under such circumstances, it is impossible to use the method having a yield of 90% for forming the bumps in view of cost and time required for the research.
The inventors of the present invention have found that a cause for lowering the yield of the products obtained by the transfer method resides in the formation of bumps prior to the transfer.